1. Engineering Research Center of Grassland Industry, Ministry of Education, Key Laboratory of Grassland Livestock Industry Innovation, Ministry of Ministry of Agriculture and Rural Affairs, Gansu Tech Innovation Center of Western China Grassland Industry, National Demonstration Center for Experimental Grassland Science Education, State Key Laboratory of Grassland Agro-ecosystems, College of Pastoral Agriculture Science and Technology, Lanzhou University, Gansu Lanzhou 730020, China